This invention relates to the recovery of oil from subterranean oil reservoirs and more particularly to improved waterflooding operations involving the injection of a viscous aqueous liquid formulated through the use of a mixture of a C.sub.5 -C.sub.8 aliphatic alcohol and an amphoteric surfactant.
In the recovery of oil from oil-bearing reservoirs, it usually is possible to recover only minor portions of the original oil in place by the so-called primary recovery methods which utilize only the natural forces present in the reservoir. Thus, a variety of supplemental recovery techniques has been employed in order to increase the recovery of oil from subterranean reservoirs. The most widely used supplemental recovery technique is waterflooding which involves the injection of water into the reservoir. As the water moves through the reservoir, it acts to displace oil therein to a production system composed of one or more wells through which the oil is recovered.
Various chemical additives may be employed in the injected water. For example, the injected water may contain surface-active agents which effect a reduction in the oil-water interfacial tension, thus enhancing the microscopic displacement of the oil by the water. Another widely used technique involves the addition of thickening agents which increase the viscosity of the injected water, normally to a value at least equal to that of the reservoir oil, in order to arrive at a favorable mobility ratio between the oil and water and increase the macroscopic displacement efficiency of the waterflood. Typically such viscosifiers or mobility control agents have taken the form of biopolymers such as the bacterial biopolymer available from the Kelco Company under the trade name "Kelzan" or synthetic polymers such as the partially hydrolyzed polyacrylamides available from the Dow Chemical Company under the trade name "Pusher" chemicals.
An alternative to the use of polymeric thickening agents involves the injection of a thickened aqueous surfactant solution as disclosed in U.S. Pat. No. 4,042,030 to Savins et al. In this process, at least a portion of the injected water is thickened through the use of an alkylaryl sulfonate in combination with a C.sub.4 -C.sub.6 aliphatic alcohol having a hydrocarbon chain length of at least 3 carbon atoms. The action of the surfactant-alcohol system is increasing the viscosity of the water is dependent upon the relative concentrations of the sulfonate surfactant and the alcohol and is effective at relatively low salinities. The alcohol is present in a concentration such that the ratio of the surfactant to the sum of the amount of the surfactant and alcohol is within the range of 0.3-0.8. The salinity of the alcohol-surfactant solution ranges up to 3.0 weight percent. The thickened aqueous solution of surfactant and alcohol may be preceded by a relatively low viscosity surfactant slug. Another surfactant waterflooding process involving the injection of a thickened aqueous surfactant solution is disclosed in U.S. patent application Ser. No. 819,805, filed July 28, 1977, by Savins et al. This application discloses the injection of a viscous surfactant slug having certain relative concentrations of a petroleum sulfonate having a relatively broad molecular weight distribution and a synthetic sulfonate having a relatively narrow molecular weight distribution which interact synergistically to thicken the aqueous liquid for mobility control purposes. The salinity of the thickened aqueous solution may range up to 4.0 weight percent. The thickened aqueous solution may optionally contain a water-soluble C.sub.3 -C.sub.6 aliphatic alcohol.
Thus far the surfactants used predominantly in decreasing the oil-water interfacial tension have been petroleum sulfonates and synthetic alkyl or alkylaryl sulfonates. While these surfactants are extremely effective in reducing the interfacial tension to desired low values within the millidyne per centimeter range, their usefulness is limited since they lack stability in the so-called "high brine" environments. These surfactants tend to precipitate in the presence of monovalent salts such as sodium chlorides at concentrations in excess of about 2 to 3 weight percent and in the presence of divalent metal ions such as calcium and magnesium ions at concentrations of about 50 to 100 parts per million and above.
In view of the limitations thus imposed on the use of the petroleum sulfonate type surfactants, various amphoteric surfactants which are stable in high brine environments have been proposed for use in surfactant waterflooding. Thus, U.S. Pat. No. 3,939,911 to Maddox et al. discloses a surfactant waterflooding process in which a sulfonated betaine such as a C.sub.12 -C.sub.24 alkyl amido C.sub.1 -C.sub.5 alkyl dimethyl ammonium propyl sulfonate is employed in conjunction with an alkyl or alkylaryl sulfonate and a phosphate ester sulfonate. The multicomponent surfactant system disclosed in Maddox et al. is said to tolerate polyvalent ion concentrations from about 200 to about 14,000 parts per million.
Another surfactant waterflooding process employing certain amphoteric quaternary ammonium carboxylates is disclosed in U.S. Pat. No. 4,076,743 to Koch et al. The amphoteric surfactants employed in this process are the reaction products of certain amino or diamino alkanols quaternized with a monohalocarboxylic acid having from 1 to 5 carbon atoms. These amphoteric carboxylate surfactants are said to be effective in highly saline waters.
Yet a further surfactant waterflooding process employing amphoteric surfactants is disclosed in U.S. application Ser. No. 815,740, filed July 15, 1977, by Stamoulis Stournas. In this process, the amphoteric surfactants employed are certain hydrocarbyl quaternary ammonium sulfonates or carboxylates. These surfactants are effective in reducing oil-water interfacial tensions in the presence of highly saline brines and preferably are employed in low concentrations in a surfactant slug injected in a relatively large pore volume amount.